The use of ozone in cooling waters, particularly recirculating cooling waters has been recently touted as an answer to control microbiological growth in these recirculating waters, and is also now being touted as a sole treatment including possible inhibition of corrosion and for scale inhibition.
A typical ozone injection system for cooling water systems includes a source of dry, clean compressed air or oxygen which is passed through an ozone generator.
Depending on the type of ozone generator used, a stream containing 1-30% ozone in air is generated. Ozone is typically produced in a gas phase either by exposing the gas phase to electrical energy in the form of a corona discharge, by electrochemical processes or through other means.
Under current practice, ozone is introduced into an aqueous system by passing a side stream of the system through a gas/liquid contactor in which ozone is transferred from a gas phase to the aqueous phase (contactor loop).
This gas stream is then passed into a contractor in which the ozone laden gas stream is intimately mixed with an aqueous stream (the contactor loop). This aqueous contactor stream is generally a side stream loop of the recirculating system. The same principle would apply to contacting cooling system make-up water and the term sidestream contactor loop as used herein is meant to also apply to the introduction of ozone into cooling system makeup water. Ozone is dissolved in the recirculating system loop (contactor loop) from the gas stream resulting in an ozone rich aqueous stream which is then reinjected into the cooling waters. The ozone concentration in the aqueous sidestream ozone contactor loop can be quite high. Since dissolved ozone is quite unstable and reactive, significant amounts of ozone can be lost in the contactor loop. Coppinger et. at., Ozone Treatment of Cooling Water:Results of a Full-Scale Performance Evaluation, presented at the 1989 Cooling Tower Institute Annual Meeting, New Orleans, La., Jan. 23-25, 1989 demonstrated that 88% of the ozone produced by the generator can be lost in the contactor loop. Efforts have been made to increase the effectiveness of ozone as a treating agent for alkaline cooling tower waters. This was accomplished by reducing the pH of the solutions in contactor loops which supply ozone to cooling tower waters with an acidic substance in Johnson et al., U.S. Pat. No. 5,415,783.
Normally, certain additives are added to cooling waters, particularly those cooling waters that have characteristics that might lead to hardness precipitates such as calcium and/or magnesium carbonate precipitates, to prevent these precipitates from accumulating and depositing on heat transfer surfaces, fouling these surfaces and contributing to lost energy efficiency in the process. Since ozone is such a strong oxidizing agent, there are many references to its reacting with organic materials, including agents purposefully added as scale inhibitors, to degrade organics. Such degradation eliminates any scale inhibiting characteristics that might be present when scale inhibitor chemicals are being used.
Examples of these teachings include a teaching by Ikemizu, et. al., Chemical Engineer Commun., 34(1-6), 77-85, 1985, wherein water soluble polymers are taught to be degraded by ozone and that the degradation rate is defined by change in the weight average molecular weight per unit time. Ikemizu teaches that the rate of degradation for poly(oxyethylenes) was proportional to 1.5 to 2.0 powers of the molecular weight and was 20 to 60 times higher than the rate of degradation of poly(acrylamides). He taught that the degradation rate of poly(sodium acrylate) was proportional to the 2.0 power of the molecular weight.
Hanasaki, in Kankyo Gijutsu, 13(11), 817-20, (1984), taught that poly(acrylamide) was ozonized to remove it from waste waters. His observations indicated that chain severing occurred in a random fashion and that ozonation produced a carbonyl(aldehyde), carboxylate functionalities and ketone groups. Although he states that the amide groups in poly(acrylamide) were not themselves attacked by ozone, these teachings and others, such as Imamura, et. al., in the Journal of Applied Polymer Science (25(6), 997-1105, (1980), taught destruction of polymers and UV light acceleration of ozonation of water soluble polymers, particularly polyethylene glycol, polyacrylamide, and poly(vinyl alcohol).
In general, chain cleavage of polymers was observed in the presence of ozone, and this was accelerated in the presence of UV irradiation. Various products were observed in the ozonation reaction including formaldehyde and the presence in oligomers of ketones, carboxylic acids, and terminal aldehydes. (See for example Suzuki, et. al., Journal Applied Polymer Science, 24(4), 999-1006, (1979.)
It is well known that ozonation causes polymer degradation containing various water soluble polymers. (See for example, Morooka, S. et al., Proc. Pacific Chem. Eng. Congress 3rd, 289-294 (1983).)
It would have appeared then from these various treatments appearing in the prior art that water soluble polymers traditionally used to maintain calcium and other hardness components in solution in recirculating cooling waters would suffer a negative fate in the presence of ozone. Ozone would be expected to degrade these polymers to the point where their usefulness would no longer exist for the purpose of maintaining calcium carbonate in solution or suspension and preventing scale formation on heat transfer surfaces in contact with the waters containing hardness. Therefore, the ability to increase the stability of water-soluble polymeric treating agents in the presence of ozone as detailed in this invention is of great utility.
Ozone has been utilized for the bleaching of pulp in the pulp and paper industry. Since ozone is known to be an indiscriminant oxidizer of wood pulp, attempts have been made to identify conditions wherein ozone-lignin interactions are maximized and ozone-carbohydrate interactions are minimized. It has been disc additives, such as methanol and dimethyl sulfoxide can "protect" cellulose during ozone delignification. Liebergott, N.; Skothos, A.; van Lierop, B. The Use of Ozone in Bleaching Pulps, 1992 Environmental Conference, TAPPI Proceedings, 1992, p. 1105-1127. However, this finding represents a conclusion only that certain organic additives may protect one organic component (cellulose) preferably to another (lignin), not that a wide variety of organic compounds may be protected successfully. By contrast, this invention demonstrates that classes of organic compounds such its water soluble polymeric additives or phosphonates can be protected from significant degradation in an environment which contains ozone.
Conventional approaches to the problem of ozone degradation of water treatment chemicals in cooling towers have involved the screening of treating agents to determine which were relatively more ozone-resistant. Rao, Narishma M., Towards Development of An Ozone Compatible Cooling Water Treatment, NACE Conference 1994, Paper No. 469. Screening techniques were also used by Khambatta, et al., U.S. Pat. No. 5,171,451 to determine which water-soluble treatment polymers were inherently more ozone resistant, and thus more compatable treating agents in ozone-containing systems.
An analogous problem is experienced in aqueous systems which involve chlorine or bromine as biocides. Because of the reactivity of these biocides, the effectiveness of other water-treatment agents is severely reduced. Protection for specific water-treating agents such as scale inhibiting phosphonates in the presence of chlorine or bromine biocides by addition of organic sulfonamides was taught in U.S. Pat. No. 5,449,476. These, however, are believed to form halogenated organic intermediates similar to choramines and are not analogous to the present invention. In fact, methane sulfonamide was found to be ineffective in Example 2.
It is evident that past solutions to the degradative tendencies of certain biocides such as ozone involve the selection of specific additives which are more resistant, or protection of certain limited classes of additives with a specific organic reagent. Thus, an effective treatment which may protect a whole class of additives, regardless of individual ozone susceptibilities would be highly desirable. It is this end which the invention has successfully addressed.